This invention relates to an imaging laminate having between a pair of sheets a layer of frangible image-forming substance separable to the respective sheets thereof. More particularly, it relates to a laminar structure particularly adapted to separation of the sheets thereof by mechanical apparatus.
Laminar imaging materials comprising a pair of sheets and a layer of image-forming substance therebetween have been known. For example, laminar thermal imaging materials for the production of images by exposure to heat and separation of the sheets thereof have been described in U.S. Pat. No. 3,924,041 (issued Dec. 2, 1975 to M. Miyayama, et al.); in U.S. Pat. No. 4,157,412 (issued Jun. 5, 1979 to K. S. Deneau); and in International Patent Application No. PCT/US87/03249 of M. R. Etzel (published Jun. 16, 1988 as International Publication No. WO 88/04237). It will be appreciated that an image-forming substance confined between a pair of sheets will be protected against abrasion and ruboff. In addition, a laminar medium can be handled as a unitary structure, thus, obviating the requirement of bringing the respective sheets of a two-sheet imaging medium into proper position in a printer or other apparatus used for imaging of the medium material.
As is disclosed in the aforementioned International Application PCT/US87/03249, the image-forming substance of the thermally imageable medium thereof is caused to be adhered weakly to a first sheet, for example, by coating a layer of the image-forming substance onto an image-forming surface of the first sheet, the image-forming surface comprising, for example, a subcoat of polymeric material which is heat-activatable to an altered physical condition upon subjection to the heat generated by brief and intense imaging radiation. The layer of image-forming substance (e.g., pigment material in a binder therefor) is designed to fracture vertically, i.e., in a direction normal to the surface of the layer, such that, portions of the layer of image-forming substance subjected to brief and intense irradiation and rapid cooling are caused to become attached more firmly or locked to the first sheet through the influences of the heated image-forming surface. Portions not subjected to such influences and remaining weakly adhered to the first sheet are removed to the opposed and second sheet of the laminate upon separation of the respective sheets thereof after imaging.
Vertical frangibility of the image-forming layer permits the production of images of desirably high resolution and optical density. Moreover, image resolution is promoted by the adhesion of the vertically frangible image-forming substance only weakly to the image-forming surface or zone of the first sheet and is reduced by a too-strong adhesion, as a consequence of which, minute pels or portions of image-forming substance not exposed to heat may undesirably remain adhered to the first sheet and not be removed to the second sheet on separation of the sheets of the laminate. Desirably, the image-forming substance will be adhered to the first sheet sufficiently to prevent accidental dislocation (separation) from the image-forming surface or zone of the first sheet and consistently with the particular requirements of image resolution and density.
In the copending and commonly assigned U.S. patent application of William P. Tobin, U.S. Ser. No. 07/616,796, filed Nov. 21, 1990, there are disclosed certain preferred thermal imaging materials, in the form of individually sized (formatted) thermally imageable laminates. Individual (formatted) thermally imageable laminates of the type shown in U.S. patent application Ser. No. 07/616,796 can be stacked in a cassette or tray for supply to a drum or other zone of a printing apparatus and, after thermal exposure, can be separated by an automated delamination device described therein. It will be appreciated that, depending upon the degree of adhesion of the image-forming substance to the image-forming surface or zone of the first sheet, subjection of the sheets to physical stresses in the form, for example, of shock or bending, may cause premature delamination of the laminate at the weakest interface thereof, i.e., at the interface of the layer of image-forming substance and the image-forming surface or zone of the first sheet. Undesired and premature delamination can also occur during the manufacture of laminar units. Individual imageable units will, for example, typically be cut from a web of laminar material having image-forming material confined between a pair of sheets. Cutting, slitting and stamping operations used during the manufacture of laminar units can create stresses, particularly at the periphery of the units, which cause delamination. Delamination initiated at the periphery or at an edge of a unit and propagated through the expansive portion of the unit will render the unit useless and is desirably to be avoided.